Age dependent costs of reproduction in a sexually dimorphic species : male fallow deer as an example

Sammanfattning: Energy and time are two of the most important factors limiting the investment spent on reproduction for an organism. Due to this limitation, individuals have to balance between using energy for maintenance or growth and using it for reproduction. In many polygynous male mammals, reproduction has been pointed out to be the most energetically costly time of the year and in sexually size dimorphic species mortality is sex-biased toward the larger sex. Fallow deer is a polygynous, seasonally breeding species demonstrating a high level of sexual size dimorphism where males can be double the size of females. During the breeding season, males compete for females and display on the breeding site. Behaviours like fighting, vocalization and a reduction in forage intake can be very costly and males often loose a large amount of weight. In this study, I estimate the cost of reproduction by using culling data together with movement data collected from wild GPS-marked male fallow deer. I compare timing of body mass- and bone marrow fat loss with daily average movements and home range sizes between different age classes during time of rut. Mature males (≥ 4-years-old) lost on average 24% of their body mass during pre- and post-rut and had a decline in bone marrow fat of approx. 20%. Simultaneously, younger males (≤ 2-years-old) lost much less, if anything. Additionally, males 1 – 2-years-old had the highest mobility during rut while 4 year- and older ones moved least. There were no significant differences in home range sizes between mature and immature males and of all seasons was autumn the season with largest home range sizes, highly influenced by the time of rut. These results suggest that the loss in body mass is not due to a higher mobility during rut and the distinct decline in both body mass and bone marrow fat indicates a large investment into reproduction in mature males. Finally, as far as I know, there are no previous published studies ever of male fallow deer home range sizes, based on GPS-data, making this study unique from that perspective.